On Tuesday, level two restrictions were enacted in the capital of New South Wales (NSW), a state 99.9% in drought.
These could be increased to level three within months, local media report.
Temperatures are predicted to soar above 40C (104F) from Tuesday, worsening fires which have already killed six people since October.
There are now more than 100 fires in NSW and Queensland states, many of which have been burning for several weeks.
What are the water restrictions?